This invention relates to hydrophones and more particularly to an optical fiber hydrophone which is hydrostatic pressure and temperature-stable.
Heretofore acoustical transducers have been made of piezoelectric or magnetostrictive materials that function to receive acoustical signals and to convert them to an electrical signal or to convert an electrical signal to an acoustical signal. These prior-art transducers are useful throughout the different depths of the waters and have reached a stage of maturity in which further improvement in their operating characteristics have, for the most part, become limited by the available transduction materials.
Recently there has been patented U.S. Pat. No. (4,068,191) an acousto-optic modulator for optical fiber waveguides in which light passing through an optical fiber is modulated by an acousto-optic modulator which is in intimate contact with the optical fiber. Also, there has been reported in the literature single optical fiber interferometric acoustic sensors in which the optic fiber is in a straight line or in a coil. Suchsystems have been reported in the following articles: (1) "Optical Fiber Acoustic Sensor," by J. A. Bucaro, H. D. Dardy, and E. F. Carome, Applied Optics, Vol. 16, No. 7, pps. 1761-1762, July 1977. (2) "Fiber-optic Hydrophone," J. A. Bucaro and H. D. Dardy, J. Acoustical Society of America, Vol. 62, No. 5, pp 1303-1304, November 1977. (3) "Fiber-optic Detection of Sound," By J. H. Cole, R. L. Johnson and P. G. Bhuta, J. Acoustical Society of America, Vol. 62, No. 5, pp 1136-1138, November 1977. (4) "Single Fiber Interferometric Acoustic Sensor," by J. A. Bucaro and E. F. Carome, Applied Optics, Vol. 17, No. 3, pp 330-331, Feb. 1, 1978.
It is well known in the prior art that, when an acoustic wave propagates in a medium, the periodic variations in pressure cause corresponding periodic variations in the optical index of the medium. Thus, if a light beam is directed through a straight optic fiber or a coil and the optic fiber is subjected to an acoustic wave, the index of refraction of the optic fiber will be changed. As the index of refraction of the optic fiber changes, the phase of the light beam traversing the optic fiber will change. This phase change can be detected by an optical interferometer system in which a light beam is split into two equal path lengths, one path interacting with the acoustic field while the other is retained outside the field as a phase reference. The above-listed publications set forth such systems in which optical fibers are used to conduct a light beam in and out of the acoustic field. As the acoustic waves change the index of refraction of the optic fiber, the light passing through the optic fiber changes its phase. This phase change is proportional to the change in the index of refraction of the optic fiber, which is proportional to the incident acoustic wave. Therefore, the phase change in the light is a measure of the acoustic field incident on the optic fiber.
It is well known that the pressure and temperature of the surrounding water will also change the index of refraction of an optic fiber. Therefore, a light beam passing through an optic fiber is affected by pressure and temperature. In the prior-art systems, only the optic fiber within the medium is affected by the surrounding pressure and temperature. This leads to an inaccurate measure of an acoustic field due to the index-of-refraction change resulting from the pressure and temperature differences. This invention overcomes the problem by subjecting the sensor coil and reference coil to the same pressure and temperature conditions so there are no differences due to pressure and temperature.